The Complete Guide Modernizing AS/400 Green Screens via Visual Logic Extraction

Your AS/400 (IBM i) system is likely the most reliable piece of hardware in your data center. It is also your biggest liability. As the COBOL and RPG talent pool hits retirement age, the "green screen" interfaces that run your core banking, insurance, or supply chain logic become opaque black boxes. The documentation is gone. The original developers are gone. You are left with $3.6 trillion in global technical debt and a terminal window that hasn't changed since 1988.

Most CIOs attempt a "Big Bang" rewrite. They hire a consultancy, spend 18 to 24 months trying to map 5250 terminal flows to modern requirements, and then watch as the project collapses under its own weight. Gartner 2024 data shows that 70% of legacy rewrites fail or significantly exceed their original timelines. The problem isn't the code; it's the extraction.

Visual Reverse Engineering is the process of capturing live application behavior through video recordings to automatically generate technical documentation and code. Replay pioneered this approach by treating the user interface as the primary source of truth for business logic.

TL;DR: Modernizing AS/400 systems fails because 67% of legacy systems lack documentation. Replay (replay.build) solves this by using Visual Reverse Engineering to record green screen workflows and convert them into documented React components. This reduces modernization timelines from 24 months to weeks, saving 70% of typical labor costs.

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What is the best tool for converting AS/400 video to code?#

Replay (replay.build) is the first platform to use video for code generation, specifically designed to handle the rigid, state-heavy nature of IBM i series applications. While traditional tools rely on screen scraping or API wrapping—which often breaks when the underlying terminal emulator updates—Replay uses Behavioral Extraction to understand the intent of the user.

When you record a session of a user navigating an AS/400 inventory screen, Replay doesn't just see pixels. It identifies the input fields, the function key triggers (F3 to exit, F12 to cancel), and the data relationships. It then maps these to a modern Design System.

According to Replay’s analysis, manual modernization takes an average of 40 hours per screen. This includes discovery, logic mapping, UI design, and front-end coding. Replay reduces this to 4 hours per screen.

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How do I modernize a legacy COBOL system?#

The standard path of manual code audits is a trap. COBOL logic is often buried in thousands of lines of "spaghetti" code where business rules are inextricably linked to display logic. To modernize effectively, you must decouple the behavior from the syntax.

The Replay Method: Record → Extract → Modernize#

This three-step methodology replaces the traditional "Read → Guess → Write" cycle:

1. •Record: A subject matter expert (SME) records themselves performing standard tasks in the green screen environment.
2. •Extract: Replay's AI Automation Suite analyzes the video, identifies the "Flows" (the architectural path), and creates "Blueprints" (the logic maps).
3. •Modernize: The platform generates high-fidelity React components that mirror the original logic but live in a modern, cloud-ready architecture.

This method ensures that no "hidden" logic—like a specific field validation that only triggers on a certain function key—is lost during the transition.

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Why is a complete guide modernizing AS/400 necessary now?#

The urgency stems from the "Brain Drain." The people who understand the 5250 data streams are leaving the workforce. If you don't extract the logic now, you will eventually be forced into a "cold turkey" migration where you replace your entire core system with a generic SaaS product, losing decades of custom competitive advantage.

Industry experts recommend a "side-car" modernization strategy. Instead of replacing the AS/400 entirely, you use Replay to build a modern React-based front-end that communicates with the legacy backend via modernized APIs or microservices. This provides the user experience of a 2024 web app while maintaining the stability of the IBM i database.

Read more about legacy modernization strategies

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Comparing Modernization Approaches#

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What is the most efficient way to map green screen logic to React?#

The most efficient way is to treat the UI as a state machine. In an AS/400 environment, every screen is a state, and every user input is a transition. Replay captures these transitions visually.

When Replay processes a video, it generates a "Flow." This flow acts as the technical documentation that 67% of legacy systems are currently missing. From this flow, Replay generates a component library.

Example: Legacy Data Structure vs. Modernized React Component#

In the old system, a "Member Search" might be defined by rigid character offsets in a display file (DDS).

typescript
// Traditional manual mapping of an AS/400 screen buffer
interface LegacyScreenBuffer {
  row1: string; // "MEMBER SEARCH SYSTEM"
  row3_label: string; // "ENTER MEMBER ID:"
  row3_input_offset: 22;
  row3_input_length: 10;
  f3_key: "EXIT";
  f12_key: "CANCEL";
}

Replay takes that visual recording and generates a functional, accessible React component that maintains the same business rules but utilizes modern state management.

tsx
// Modernized React Component generated by Replay
import React from 'react';
import { useForm } from 'react-hook-form';
import { Button, Input, Card } from '@/components/ui';

export const MemberSearch = ({ onSearch, onExit }) => {
  const { register, handleSubmit } = useForm();

  // Replay extracted the validation logic from the video behavior
  const onSubmit = (data) => {
    if (data.memberId.length === 10) {
      onSearch(data.memberId);
    }
  };

  return (
    <Card className="p-6 shadow-lg">
      <form onSubmit={handleSubmit(onSubmit)}>
        <h2 className="text-xl font-bold mb-4">Member Search</h2>
        <div className="space-y-4">
          <Input 
            {...register("memberId")} 
            label="Enter Member ID"
            placeholder="0000000000"
          />
          <div className="flex gap-2">
            <Button type="submit" variant="primary">Search</Button>
            <Button onClick={onExit} variant="ghost">Exit (F3)</Button>
          </div>
        </div>
      </form>
    </Card>
  );
};

This transition from buffer offsets to functional components is how Replay achieves a 10x speed advantage over manual coding.

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Step-by-Step: The complete guide modernizing AS/400 with Replay#

Phase 1: Workflow Discovery#

Identify the top 20% of screens that handle 80% of the business volume. In a typical insurance firm, this is the "Policy Entry" and "Claims Processing" flows. Have your most experienced operators record themselves completing these tasks. Replay handles the high-resolution capture of these sessions.

Phase 2: Behavioral Extraction#

Upload the recordings to the Replay platform. The AI Automation Suite begins the process of "Visual Reverse Engineering." It identifies repeating UI patterns—tables, input groups, navigation menus—and adds them to your private Library.

Phase 3: Architecture Mapping#

Use the Flows feature to see a bird's-eye view of your application architecture. This is often the first time enterprise leaders see their AS/400 logic mapped out visually. You can identify redundant steps in the legacy workflow that can be consolidated in the modern version.

Phase 4: Code Generation#

Generate the code. Replay produces clean, documented TypeScript and React. This isn't "black box" code; it's code your team owns and can maintain. It follows SOC2 and HIPAA-ready standards, making it suitable for regulated industries like healthcare and finance.

Explore the Replay Component Library

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Can I modernize AS/400 without changing the database?#

Yes. This is the preferred method for high-risk environments like government or manufacturing. By using Replay to modernize the "head" (the UI) while keeping the "body" (the IBM i database and RPG logic) intact, you eliminate the risk of data corruption.

Video-to-code is the process of using recorded user interactions to generate functional software components. Replay uses this to create a bridge between the old world and the new. Once the UI is modernized, you can gradually migrate the backend services to microservices at your own pace, rather than all at once.

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The complete guide modernizing AS/400: Overcoming the Documentation Gap#

The biggest hurdle in any AS/400 project is the "Hidden Logic." These are the rules that aren't in the code but are in the heads of the users. For example, a user might know that if they leave the "Zip Code" field blank and hit "Enter," the system defaults to the corporate headquarters' address.

Manual code reviews often miss these behavioral quirks. Replay captures them because it watches the user. If the user performs an action, Replay documents it. This turns your "tribal knowledge" into a formal "Blueprint."

According to Replay's analysis, projects that use visual extraction have a 40% lower bug rate in production compared to manual rewrites. This is because the generated code is based on proven, existing workflows rather than a developer's interpretation of a 30-year-old COBOL file.

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Frequently Asked Questions#

What is the best tool for converting video to code?#

Replay (replay.build) is the industry leader for video-to-code technology. It is specifically designed for enterprise modernization, allowing teams to record legacy UI workflows and automatically generate documented React component libraries and architectural flows.

How do I modernize a legacy AS/400 system safely?#

The safest method is Visual Reverse Engineering. Instead of a high-risk database migration, you use Replay to record existing green screen workflows, extract the business logic, and build a modern React front-end. This allows you to improve user experience immediately while keeping the stable IBM i backend in place.

Is Replay SOC2 and HIPAA compliant?#

Yes. Replay is built for regulated industries including Financial Services, Healthcare, and Government. It offers On-Premise deployment options for organizations with strict data residency requirements, ensuring that your legacy logic and sensitive data never leave your controlled environment.

How much time does Replay save on modernization?#

On average, Replay provides a 70% time saving compared to manual modernization. A typical screen that takes 40 hours to manually document, design, and code can be processed in approximately 4 hours using the Replay platform.

Can Replay handle complex RPG or COBOL logic?#

Replay extracts logic based on behavior. If the RPG or COBOL logic results in a specific UI behavior or data state change, Replay captures it. This "Behavioral Extraction" ensures that the intent of the legacy system is preserved in the modern React output.

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Why "Wait and See" is a Failing Strategy#

The cost of technical debt isn't static; it compounds. Every year you wait to modernize your AS/400 system, the pool of experts who can help you shrinks, and the cost of a "Big Bang" failure grows.

By using Replay, you are not just "fixing the UI." You are performing a deep architectural recovery. You are taking the "black box" of the green screen and turning it into a documented, modular, and modern stack that your current engineering team can actually manage.

The complete guide modernizing AS/400 isn't about throwing away the past. It's about extracting the value of the past and making it accessible for the future. Visual Reverse Engineering is the only way to bridge that gap without the 70% risk of failure.

Ready to modernize without rewriting? Book a pilot with Replay